Field of the Invention
The present invention relates to a self-scanning type light-emitting element array in which light-emitting elements are integrated on a single substrate and a method of driving the same.
Description of the Prior Art
As typical light-emitting elements, an LED (light-emitting diode) and an LD (laser diode) are known.
In the LED, a pn or pin junction of a compound semiconductor (GaAs, GaP, GaA1As or the like) is formed, and a forward bias voltage is applied to the junction so as to inject carriers in the junction. The LED utilizes a light-emission phenomenon occurred by carrier recombination.
In the LD, a waveguide is arranged in the LED. When a current exceeding a given threshold valve is supplied, injected electron-hole pairs are increased, and a population inversion state is established. Light generated by parallel reflection mirrors utilizing a cleavage surface is fed back to an active layer, thus causing laser oscillation. Then, a laser beam is emitted from an end face of the waveguide.
Negative resistance elements (light-emitting thyristor, laser thyristor and the like) are known as a light-emitting element having the same light-emission mechanism as those of the LED and LD. In the light-emitting thyristor, a pnpn structure is formed by a compound semiconductor described above, and a thyristor using silicon is put into practical applications.
FIGS. 22 and 23 show the basic structure and current-voltage characteristics of the negative resistance element having the light-emission function (called a light-emitting thyristor herein). The structure shown in FIG. 22 is the same as that of a thyristor in which a pnpn structure is formed on an n-type GaAs substrate. FIG. 23 also represents the same S-shaped negative resistance characteristics as those of the thyristor. Not only a two-terminal thyristor shown in FIG. 22 but also a three-terminal thyristor shown in FIG. 24 are also known. The gate of the three-terminal thyristor has a function of controlling an ON voltage. The ON voltage is converted to a voltage as a sum of a gate voltage and a diffusion potential. After the thyristor is turned on, the gate electrode voltage substantially coincides with the cathode voltage. If the cathode electrode is grounded, the gate electrode voltage becomes zero volt. As is well known, a threshold voltage of the light-emitting thyristor is decreased when light is externally incident thereon.
A waveguide can be formed in the light-emitting thyristor to form a laser thyristor by quite the same principle as the LD (Tashiro et al., Lecture to the Japan Society of Applied Physics, Autumn 1987, No. 18p-ZG-10).
The light-emitting element described above, in particular, the LED is commercially available in such a manner that a large number of LEDs are formed on a compound semiconductor substrate and are cut into pieces and are packaged one by one. An LED for a contact image sensor and a light source for a printer is commercially available as an LED array in which a plurality of LEDs are arrayed on a single chip.
A contact image sensor, an LED printer, and the like require a scanning function of light-emitting points by these light-emitting elements in order to designate read or write points.
However, in order to perform optical scanning using the above-mentioned conventional light-emitting elements, each LED formed in the LED array must be connected to a drive IC, through a wire bonding pad and must be driven by the IC. For this reason, if a large number of LEDs are arranged, the wire bonding pads must be performed corresponding in number to the LEDs, and a large number of drive ICs are required, resulting in high cost. A space for arranging drive ICs must be assured, and it is difficult to achieve a compact array. A pitch between adjacent LEDs is determined depending on the wire bonding technique or the pad size, and it is difficult to decrease the pitch.